B737-300 INGESTS FOD AFTER ENG START-UP.

Narrative

AN INOP APU NECESSITATED AN EXTERNAL AIR START AT THE GATE. SHORTLY AFTER BEGINNING PUSHBACK; THE TUG DRIVER RPTED HE SAW A PEBBLE LIKE OBJECT FLY UP INTO THE RUNNING ENG AND FOR A MOMENT THEREAFTER; HE OBSERVED SPARKS COMING FROM THE TIPS OF THE FAN BLADES. MY FIRST INCLINATION WAS TO SHUT DOWN THE ENG; BUT THAT WOULD HAVE LEFT US ON EMER PWR; SO INSTEAD; WE SETTLED FOR LEAVING THE TUG ATTACHED AND CONTACTING MAINT CTL WHO INSTRUCTED US TO RETURN TO THE GATE. WE CONTACTED RAMP WHO WOULD NOT ALLOW US BACK INTO THE SAME GATE. RAMP CTL INSTRUCTED US TO TAXI TO ANOTHER GATE. WE TOOK A MOMENT TO BRIEF THE FLT ATTENDANTS AND PAX; PROCEEDED TO OUR ASSIGNED GATE AND SHUT DOWN. THE MAINT TECHNICIAN HAD A FEW THINGS TO SAY TO ME INCLUDING QUESTIONING MY CHOICE TO TAXI UNDER PWR TO A DIFFERENT GATE AND INSISTING THAT THERE WAS NO WAY AN IDLING ENG COULD SUCK UP AN OBJECT FROM THE GND. IN SPITE OF HIS SECOND ASSERTION; THERE APPEARED TO BE DAMAGE TO 4 OF THE FAN BLADES AND 4 OR 5 HOLES IN THE SOUND BAFFLING IN FRONT OF THE FAN.